1. Field of the Invention
This invention relates to recovery of olefins from pryolyzed hydrocarbon gases and especially relates to recovery of olefins from cracked hydrocarbon gases by absorption with a preferential physical solvent.
2. Review of the Prior Art
Olefins have a wide variety of petrochemical uses. Ethylene is a principal building block of the petrochemical industry. Its largest single use is the conversion to low-and high-density polyethylenes, which are used in packaging, communications, construction, automotive, manufacturing of home appliances, and many other industries. Other major uses include oxidation to ethylene oxide and chlorination to ethylene dichloride.
Olefins are generally produced by thermally or catalytically cracking gaseous or liquid hydrocarbons to make cracked gases. Three general methods of separating or concentrating the components of cracked gases for the recovery of ethylene of moderate and high purity have been available for many years. One involves use of a solid adsorbent such as charcoal or silica gel. The other two utilize fractional distillation in two variations, the first being a straight low-temperature fractionation process and the second involving absorption into liquid having low vapor pressure, thereby avoiding very low temperatures in the fractionation system.
Low-temperature fractionation, a refinement of the stepwise liquefaction method, was used as of about the mid-1960's in the majority of ethylene plants in operation, as described in "Low Temperature Fractionation of Light Hydrocarbons", by A. W. Pratt and N. L. Foskett, Transactions of the American Institute of Chemical Engineers, Vol. 42, 1946, page 149.
In a typical modern olefin plant employing low-temperature fractionation for ethylene recovery as illustrated in FIG. 1, the predominant feedstock is composed of ethane, propane, butanes, naphtha, gas oil, condensate, and other hydrocarbons derived from petroleum cracking. These feedstocks are pretreated and cracked by conventional steam crackers. The cracked gases leave the cracking furnaces at 1500.degree. to 2200.degree. F. These gases are quenched and cooled down to about 80.degree. to 120.degree. F. at pressures less than 15 psig. The above feedstocks may also be catalytically cracked under appropriate operating conditions.
Depending upon the feedstock and the severity and selectivity of cracking, the cracked gases comprise hydrogen, methane, carbon monoxide, carbon dioxide, acetylene, ethylene, ethane, methyl acetylene, propadiene, propylene, propane, butadienes, butenes, butanes, C.sub.5 's, C.sub.6 -C.sub.8 non-aromatics, benzene, toluene, xylene, ethyl benzene, styrene, C.sub.9 -400.degree. F. gasoline, 400+.degree. F. fuel oil and water.
These gases are compressed in multi-stage compression units to pressures in the order of 400 to 600 psig. During compression, some heavier hydrocarbons and water are separated. Depending upon their composition, the separated hydrocarbons are stabilized and may be utilized as part of the feed to the fractionation train. The uncondensed gases after compression are further dried by molecular sieves or activated alumina to a water dew point of less than -150.degree. F. The cracked gases may be dried at an intermediate pressure level consistent with the interstage pressure of the multi-stage cracked gas compressor.
The dry gases are then chilled in a series of steps to cryogenic temperatures of -200.degree. F. through a complicated set of equipment utilizing extensive heat exchange and ethylene and propylene refrigeration systems. The purpose of chill-down is to separate ethylene and heavier hydrocarbons from the methane and hydrogen present in the cracked gases. The remaining stream of methane and hydrogen is further separated into hydrogen-rich and methane-rich streams within the cryogenic chill-down train. A part of the separated hydrogen stream is preferably further purified by using conventional pressure swing adsorption techniques before utilizing it for hydrogenating acetylenes to desirable products while the methane-rich stream is used as fuel gas for the steam cracking furnaces.
The separated liquid streams, containing ethylene and heavier hydrocarbons, are further fractionated in a low-to-high-pressure demethanizer (150 to 450 psig). The low-pressure demethanizer utilizes ethylene refrigerant for the overhead condenser while the high-pressure demethanizer uses low-level propylene refrigerant, sometimes in conjunction with an expander unit, for such condensation.
The specification of methane in the bottom of the demethanizer is quite stringent since methane represents an impurity when present with the ethylene product. Similarly, it is highly desirable to recover most of the ethylene present in feeds to the demethanizer in order to reduce its loss and thus minimize the need for recycling demethanizer overhead to the cracked gas compressors for additional recovery of this desirable product.
The olefin plants are known to be energy intensive. For a 1.4 billion pounds per year ethylene-capacity plant using modern available technology for 50/50 condensate and ethane/propane feedstock, the charge gas compressor requirements can vary between 36,000 and 39,000 horsepower (HP) while the corresponding requirements for the propylene refrigeration system can vary from 18,000 to 22,000 HP and those for the ethylene refrigeration system can vary from 3,900 to 8,000 HP. Thus, depending upon the technology involved and the desired product slate, the total compression energy can vary between 58,700 and 69,000 HP, thereby requiring large capital investment and representing a significant cost of operation related to energy consumption. In summary, depending upon the desired product slate, available feedstock, severity, and selectivity of cracking and separations technology utilized, the specific energy consumption of an olefins plant can vary between 5,500 and 13,500 Btu/Lb of ethylene product. This represents about 25.4% to 62.4% of the gross heating value of ethylene of 21,629 Btu/Lb.
It is important to note that the cracking system for olefin production appears to have been improved and refined to an operating state of high efficiency. Even though the steam crackers require significant amounts of fuel, since the cracking process occurs at extremely high temperatures, most of the energy expended in the cracking furnaces is recovered through extensive use of waste heat recovery equipment. For example, of the energy which is liberated by thermally cracking the hot gases, only about 3-5% is lost, and that loss is through the stacks. Most of the energy thus recovered is utilized for compressing the cracked gases and providing refrigeration for the chill-down train. Therefore, the major energy consumption in an olefins facility is associated with the chill-down and fractionation train in order to separate various components of the cracked gas stream.
According to a recent article, there are twenty-three companies that operate thirty-two olefin plants in the United States of America. This article is entitled, "New Capacity Forces Ethylene Producers to Aim for Lower Costs, Flexibility", by Ted Wett, and appeared in the Sept. 2, 1985 issue of Oil and Gas Journal, page 39. The equivalent total installed production capacity of these thirty-two plants or facilities was about 38 billion pounds per year of ethylene as of June 1985. On a worldwide basis, the total ethylene plant capacity was about 108 billion pounds per year as of about June 1985. The feedstocks used for plants or facilities was at least one of ethane, propane, butane, refinery streams, naphtha, gas oil, natural gas liquids (NGL), liquefied petroleum gas (LPG), etc.
As an example of the most modern ethylene facilities, Europe's newest ethylene crackers started production in about October 1985 in a $1.6 billion plant at Mossmoran, Scotland with six cracking furnaces operating on ethane as feedstock. It is the first cracking facility known to use a gas turbine to drive the process-gas compressor and the first to employ the turbine exhaust gas as preheat combustion air for the furnaces. As the other side of the ethylene capacity coin, about 600,000 metric tons per year of cracking facilities are being shut down in England and Germany because of its competitive effects.
Problems with imbalance of capacity and demand have been plaguing the olefin production industry for many years. Nevertheless, new plants of steadily increasing size and complexity have been built that incorporate the newest technologies. In some instances, these plants exist for nationalistic reasons, whether or not they are profitable. In most cases, however, the plants come into existence because of cheap feedstock sources or because incorporating the newest technologies can enable a plant to compete successfully with older plants and capture a share of the existing market and/or of its anticipated expansion. As the newer plants come onstream, the older plants may have to be shut down, reduced in capacity, or reduced in production costs.
Among the changes that can be instituted to effect cost reduction are changing the crackers to provide flexibility for processing a wide variety of feedstocks according to market conditions. Another cost-reducing step is to revise existing facilities in order to optimize feed and energy requirements, possibly including co-generation systems with various units. A third step is to make full use of computer control in order to maximize plant operating efficiency. A fourth step is to replace obsolete plants within a facility with more efficient plants using improved technology.
The next stage for improving efficiency of olefin production is believed to be replacement of low temperature plants for recovery and separation of olefins from cracked gases. Although the industry has docilely continued to utilize low-temperature fractionation for many years after discarding solvent absorption systems such as the Kniel process, it pays a relatively high price therefor because low-temperature fractionation requires extreme dehydration and achieves it by using energy-intensive ethylene and propylene refrigerating systems.
As of about 1948, according to page 651 of "Petroleum Refinery Engineering" by W. L. Nelson, third edition, McGraw-Hill Book Company, Inc., New York, N.Y., 1949, 830 pages, ethylene was fundamentally important in making many chemicals such as alcohol, ethylether, styrene, ethylene glycol, and tetraethyl lead. A total daily plant capacity of 3,370,000 lbs per day was being built or was in operation in 1948, not including plants producing ethylene for hydration to alcohol. The plants were mainly of the thermal decomposition type, operating primarily on ethane and propane, in which the cracked gases were fed to an absorber-stripper column producing fuel gas as overhead and a rich solvent which was fed to a de-ethanizer column as the first component of a fractionation train, as shown in FIG. 1 of U.S. Pat. No. 2,573,341 which is FIG. 2 of the drawings of this invention, as representative absorption prior art.
The absorption method is discussed for an absorption-type recovery plant in an article in Chemical Engineering Progress, by Ludwig Kniel and W. H. Slager, Vol. 43, No. 7, pages 335-342, July 1947, using the same process in its FIG. 2 that is illustrated on page 652 of the book, "Petroleum Refinery Engineering", and in FIG. 1 of U.S. Pat. No. 2,573,341.
This ethylene plant of Monsanto Chemical Co. at Texas City, Tex. was mainly used for producing ethylene and operated primarily on ethane and propane. Typical ultimate ethylene yields were 75 wt. % from ethane, 48 wt. % from propane, or 25-32 wt. % from gas oil. At a conversion per pass of about 45% when cracking propane, yields were about 16.7 wt. % of ethylene and 15.8 wt. % of propylene, once through. The ethylene was separated along with the ethane and heavier components by means of low-temperature absorption with an aromatic distillate produced in the process and containing more than 50% benzene and toluene by weight and appreciable quantities of naphthenes, among which cyclopentane and cyclohexane had been identified.
Typical analyses of three ethylene-bearing streams were given in this article: (a) a typical coke-oven gas, (b) a refinery off-gas, and (c) the effluent from a pyrolysis unit charging propane and operated to yield a maximum amount of ethylene. For these three stocks, ethylene concentration was 4-27 mol % and diluents lighter than ethylene were 94-17 mol %, thereby bracketing most commercial gases from which ethylene or ethylene+propylene might be economically recovered.
In an article in Petroleum Refiner, by Ludwig Kniel, Volume 27, No. 11, November 1948, the design of a fractionating absorber, which is essentially the same apparatus as the absorber-stripper discussed in the earlier article, is described for separating methane from ethylene in a pyrolysis gas obtained from the cracking of propane. It had been found in plant operations that the performance of such fractionating absorbers exceeded the requirements anticipated in the design as to both recovery and design purity. The reason therefor was speculated to be either due to the type of lean oil employed which contained substantial proportions of aromatics, particularly benzene, or the result of superimposed recirculation occurring between the plates where intercoolers were located.
An article in Hydrocarbon Processing & Petroleum Refiner, by Joe J. Weatherby, April 1962, Volume 41, No. 4, describes a low-temperature absorption plant which recovered 40% propane from 11 MMCFD of wet natural gas. This installation utilized a fractionator absorber having a reboiler and no intercoolers. This column not only absorbed the hydrocarbons from the gas stream but fractionated off the methane and ethane to yield a de-ethanized product. A portion of the C.sub.7 + fraction of the product was pumped over the top of this column as absorption oil. Ethylene glycol was injected into the inlet gas stream upstream of the gas exchanger and of the inlet gas chiller wherein it was cooled to 20.degree. F. with ammonia as refrigerant. A gas exchanger and inlet gas chiller removed about 1/2 of the heat of absorption outside of the absorber but they consequently lowered mean effective absorber temperature, and the oil content of the residue gas leaving the top of the column was in equilibrium with the oil at the minimum temperature of 20.degree. F., thereby minimizing oil loss.
Numerous processes are known in the solvent absorption art for olefin recovery from cracked, refinery, and synthetic gases containing these unsaturated compounds. Some processes utilize an aromatic absorption oil as a solvent within an absorber-stripper column having a reboiler. Such processes are disclosed in one or more of U.S. Pat. Nos. 2,187,631, 2,282,549, 2,308,856, 2,325,379, 2,357,028, 2,433,286, 2,455,803, 2,570,066, 2,573,341, 2,588,323, 2,708,580, 2,849,371, 3,055,183, 3,082,271, 3,349,145, 3,686,344, 4,072,604, and 4,479,812.
U.S. Pat. No. 2,573,341 of Ludwig Kniel, which issued from Ser. No. 717,264, filed Dec. 7, 1946, relates to a process for recovering olefinic hydrocarbons and particularly high purity ethylene from coke oven gas, refinery off-gas, and pyrolysis gas, having respective ethylene contents of 4.0%, 5.0%, and 27.0 mol. %, which are the feedstocks to a rectifier-absorber, also known as an absorber-stripper. The column or tower has a reboiler at its bottom and two intercoolers to remove the heat of extraction. The overhead is fuel gas, and the bottoms are fed to a de-ethanizer column, having a reboiler at its bottom and a condenser for its overhead from which a portion of the condensate is returned to the de-ethanizer column as reflux. The lean oil rate is not over 4.2 lbs per lb of feed, an amount which assures the retention of 99 mol % of the ethylene entering the rectifier absorption tower.
Absorption, exemplified by the process in FIG. 1 of U.S. Pat. No. 2,573,341, was stated on page 651 of the Nelson book to be the process mainly used for ethylene recovery as of about 1948. In the mid-1950's, a number of absorption plants were in operation which utilized refrigerated lean oils such as propane, butane, or light aromatic fractions.
As of the mid-1960's, according to page 85 of "Manufacturing Ethylene", by S. B. Zdonik, E. D. Green, and L. P. Hallee, Petroleum Publishing Co., 1970, a few absorption plants were still in operation but were decreasing in number while all of the newly built plants employed some form of low-temperature fractionation to separate ethylene from light hydrocarbons At plant capacities of about 100 million lb/year of ethylene production, refrigerated-absorption recovery plants were economically comparable to low-temperature-fractionation recovery plants.
As the size of ethylene plants increased, the large heating and cooling loads imparted to the solvent in the absorption-type plants caused them to be more uneconomical from the standpoint of operating and plant costs, even though the low-temperature-fractionation plant used a more complicated refrigeration system because of the extreme low temperature involved.
Thus it appears that a classic competitive battle occurred during the twenty years from the end of World War II to about 1965, with the winner being declared the low-temperature fractionation plant and the loser being declared the solvent absorption plant. A review of the reasons for this decision of the marketplace is in order.
Referring to FIG. 1 of U.S. Pat. No. 2,573,341 as the dominant absorption process according to Nelson, there appear to be at least seven reasons that plants using the Kniel absorption process, for recovery and separation of olefins from thermally cracked hydrocarbon gases, failed to win the competitive battle. Firstly, it should be noted that this Kniel process was not applicable to the separation of methane from hydrogen, yet such separation was needed for hydrogenating acetylene to ethylene. Hydrogen was therefore used as fuel, an economically wasteful process.
Secondly, the process appears to have been biased toward the continuous production of aromatic fractions and retention thereof in the aromatic system to the extent necessary to maintain a supply of absorber oil. This situation indicates that the losses of lean absorption oil were considerable and that such losses went to fuel use. Indeed, the cracking furnaces appear to have been designed for producing the aromatic absorber oil rather than for producing ethylene.
Thirdly, it is stated in the '341 patent that it is also essential that methane be eliminated by the absorber-stripper column, without loss of ethylene in its overhead stream, and that larger amounts of methane than anticipated not be allowed in the bottoms material from the absorber-stripper column because such larger amounts could jeopardize the operation of the de-ethanizer and ethylene fractionator by causing an inability to condense the reflux in these towers at the temperatures required to obtain the desired concentrations in the overheads from each of these towers. According to an Amendment filed on Jan. 8, 1951 during the prosecution of Ser. No. 717,264, "It was found that in the design of such plant there was a tendency to either lose ethylene overhead in the fuel gas line or to accumulate methane in the bottoms of the absorber, which contaminated the ethylene product." The solution apparently adopted and claimed U.S. Pat. No. 2,573,341, as shown in its FIG. 2, was to add the demethanizer column, but the demethanizing column is superfluous if the absorber-stripper column is properly designed.
Fourthly, about 96% of the methane was rejected, thereby indicating that 4% of contained methane was present with the ethylene product, a figure that is substantiated by an ethylene purity of 97%. Purity requirements have steadily increased, however, and there may have been increased economic burdens for meeting these requirements that the Kniel-type absorption process was not able to meet.
Fifthly, it appears that the absorber-stripper column was installed primarily for reducing cracking requirements, by preliminary removal of hydrogen and methane in the refinery off-gas feedstocks, instead of for recovering and separating cracked gases. Furthermore, the lean oil fraction which was utilized in the absorber-stripper column consisted of 54.0% of benzene and toluene and 17.6% of pentanes and lighter hydrocarbons which are significantly lighter than the usual lean oil.
Sixthly, the specification also indicates that the process was limited to ethylene as its product and was not flexible enough to provide additional desirable byproducts such as propylene, which was sent to the heaters for cracking, and acetylene, which was totally ignored in the process and also recycled. Further, other unreacted paraffins and olefin fractions heavier than ethylene were recycled to the cracking furnaces in order to make the aromatic distillate to be used as the absorber oil, even though a market existed for the propylene and undoubtedly for the butadiene at that time, thereby creating another economically wasteful aspect of the process.
As the seventh reason, the degrees of freedom for operating the Kniel process were restricted by using only temperatures and lean oil flow rates. Pressure, however, was also available as a control factor. Neglecting to use it may have led to some of the problems of the process.
Three Kniel process absorber plants had been built by the Lummus Company as of Feb. 1, 1950, according to an affidavit of Ludwig Kniel that was filed during the prosecution of Ser. No. 717,264. If confronted with high losses of absorber oil in one of these absorber plants, the designers could have taken any of several curative routes.
One such route would have been to use a higher molecular weight oil, such as the heavy ends produced in the oil-gas separators in FIG. 1 of U.S. Pat. No. 2,573,341 or the heavy ends rejected by the rerun tower, but it was known that fewer absorber molecules would then be available for absorption, thereby leading to a lower loading capacity. Lower loading capacities meant a larger solvent inventory, larger pumps, more reflux, and the like. This route was apparently rejected.
Another route would have been to adjust to the loss situation by in-plant production of absorber oil. It is known, for instance, that low partial pressure and high steam ratio maximizes olefin production and that high partial pressure and low steam ratio, plus long residence time, maximizes aromatic production. Thus it would have been a relatively simple matter to have made adjustments in the operation of the cracking furnaces for producing a desired amount of aromatics to replenish losses of absorber oil. This route was apparently taken. However, the process would have thereafter been locked into aromatic production with concomitant diminishing of ethylene production and apparent inability to compete with low-temperature fractionation processes.
The Kniel absorption process consequently seems to have suffered from the following problems:
A. wasteful burning of hydrogen, propylene, and acetylene; PA1 B. excessive loss of aromatic absorber oil and replacement with distillate produced in the process, thereby decreasing ethylene production; PA1 C. an apparent need for an additional column (the demethanizer) for separating methane from ethylene in order to be able to produce ethylene of sufficiently high purity; and PA1 D. inability to economically meet increased demand for ethylene purity and recovery.
The parent patents and applications relating to the Mehra Process have utilized preferential physical solvents for recovering hydrocarbon gas liquids from natural gas streams in two embodiments: extractive flashing and extractive stripping.
The extractive flashing embodiment of the Mehra Process comprises extracting the natural gas streams with a preferential physical solvent, flashing the rich solvent, and compressing, cooling, and condensing the desired C.sub.2 + hydrocarbons, as disclosed in U.S. Pat. Nos. 4,421,535, 4,511,381, 4,526,594, and 4,578,094 and in Ser. Nos. 758,351 and 759,327. The condensed hydrocarbons are then selectively demethanized to retain selected C.sub.2 +, C.sub.3 +, or C.sub.4 + hydrocarbons, and the removed C.sub.1, C.sub.1 +C.sub.2, or C.sub.1 +C.sub.2 +C.sub.3 hydrocarbons are recycled to the extraction step. The extractive flashing embodiment is described on pages 7 and 8 of the Oct. 14, 1985 issue of the Gas Processors Report, P.O. Box 33002, Tulsa, Okla. 74153.
The extractive stripping embodiment of the Mehra Process, as disclosed in Ser. Nos. 784,566, 808,463, 828,988, and 828,996, comprises contacting a raw gas stream with a preferential physical solvent in an extractive stripping column comprising an upper extraction section and a lower stripping section. The gas enters the column below the extraction section and flows upwardly where it contacts lean preferential physical solvent which, after entering the extraction section at the top of the column, flows downwardly and countercurrently to the upwardly moving gas stream. The contact takes place over mass transfer surfaces, such as packing or distillation trays. The solvent leaving the bottom of the extraction section is rich in C.sub.1 and heavier hydrocarbons.
This C.sub.1 +-rich solvent enters the stripping section of the column and flows downwardly, where it comes in contact with the upward-flowing stripped vapors from the reboiler at the bottom of the column. The stripped vapors consist primarily of undesired components, such as methane if the desired objective is recovery of ethane and heavier hydrocarbons, or methane and ethane if the desired objective is the recovery of propane and heavier hydrocarbons, and so forth, depending upon the desired recovery objectives.
Returning to cost reduction possibilities for olefin facilities, it appears that a likely place therefor is the cryogenic chilling train in which hydrogen, methane, and ethylene fractions are separated. A system that can effect such separation via a less energy-intensive route can be useful in many existing olefin plants.
More specifically, the demethanizer column is an especially high-intensive energy user in a low-temperature fractionation plant because it requires extremely cold temperatures. An improved process that can eliminate the demethanizer column is accordingly needed.
A third area inviting improvement is the production of high-purity ethylene, at ethylene recovery levels better than 98% and at ethylene purities beyond 99.9%, in a more economical fashion by reducing energy consumption at a reasonable cost.